Police have described the death of a man who hit his head diving off a river cliff in Manawatu this evening as a tragic accident.

The 33-year-old man had been swimming with a group in the Pohangina River near the Raumai Reserve bridge when the group decided to climb a rope on a surrounding cliff at about 5.45pm, Palmerston North police Sergeant Kaine Austin said.

''They've decided to jump off the cliff and as the man has done so he has collected his head on a rock on the way down.''

The fall from the top of the cliff to water level is about five metres.

The reserve area was crowded with people cooling off in the river after the hottest day in Manawatu since January 2011. Bystanders had rushed to help the man, Austin said.

''A number of members of the public have gone to his aid. They brought him onto shore and have attempted to perform CPR on him, but this has been unsuccessful and unfortunately the man has died.

''We want to thank all of the people who were assisting. They did everything they possibly could have done.''

Two ambulances, two fire trucks and police officers arrived at the scene shortly after, with St John medics taking over attempts to revive the man.

Austin said the death, which has been referred to the coroner, should serve as a cautious reminder to those swimming in rivers.

''It appears to be a tragic accident but people need to be careful when they are diving into an unfamiliar piece of water.

''They need to make sure they are not going beyond their means.''

Travis Thomson arrived at the river with a group of friends as the incident was unfolding.

Thomson, who is a regular at the area, said there used to be warning signs next to the rope but they were not there today.

The cliff-face had slipped during the 6.3 magnitude earthquake and another slip had occurred just minutes after they had arrived today, he said.